1. Field of the Invention
The present invention relates to spacecraft attitude determination systems and more particularly to a system that provides pitch, roll, and yaw attitude control for an orbiting satellite using Global Positioning System (GPS) signals as input during operation.
2. Prior Art
A major issue encountered during the development of a suitable GPS Attitude Receiver for spacecraft is the need for accurate estimates of the electrical delay properties of the system, such as phase lag in the connecting cables between the antenna and receiver, which delays are referred to as "line-biases".
Existing works relating to the GPS, include:
1) KNIGHT, D., "A New Method of Instantaneous Ambiguity Resolution", Presented at the Institute of Navigation 7th International Technical Meeting, Salt Lake City, Utah, Sep. 20-23, 1994; PA1 2) COHEN, C., "Attitude Determination Using GPS.", Ph.D. Dissertation, Stanford University, December 1992; PA1 3) PARKINSON, B., ET AL, ed., "Global Positioning System: Theory and Applications", Vols. I and II, American Institute of Aeronautics and Astronautics, 1996; and PA1 4) BROCK, J. K., ET AL, "GPS Attitude and Orbit Determination for Space", ION GPS-94 Conference Proceedings, Salt Lake City, Utah, Sep. 20-23, 1994. PA1 5) BROCK, J. K., ET AL, "GPS Attitude Determination and Navigation Flight Experiment", ION GPS-95 Conference Proceedings, Palm Springs, Calif., Sep. 12-15, 1995; PA1 6) FULLER, R. A., ET AL, "Spacecraft Guidance and Control with GPS Tensor", Presented at 19th Annual AAS Guidance and Control Conference, Breckenridge, Colo., Feb. 7-11, 1996; and 7) FULLER,R. A., ET AL, "GPS Attitude Determination From Double Difference Differential Phase Measurements", ION GPS-96 Conference Proceedings, Kansas City, Mo., Sep. 17-20, 1996. PA1 .DELTA..phi.ij=the Differential Phase Measurement with respect to the Reference or Master Antenna signal; PA1 i=GPS Channel, 1, 2, . . . , 9; PA1 j=Antenna Baselines, 1, 2, 3; PA1 Si.sup.T =the GPS Satellite Line of Sight (LOS) vector in Earth Fixed Earth Centered, ECEF, coordinates; PA1 A.sup.T =the Transformation Matrix, ECEF from Body Coordinates; PA1 Xj=the Antenna Baseline Vector in Body Coordinates; PA1 kij=the Differential Integer Ambiguity; PA1 .beta.j=the Differential Line-Bias or electrical path length; and PA1 Vij=the Phase Measurement Noise. PA1 .theta.=Carrier phase measurement; PA1 H=Observation matrix; PA1 x=State, typically antenna position and clock bias; PA1 n=the integer combination, one per satellite and there are m satellites, numbered i=1 . . . m, with each ni=from 1. . . to m representing a carrier cycle integer and associated uncertainty bounds; PA1 .epsilon.i=phase measurement error with an assumed Gaussian probability distribution and variance Ri; PA1 a=antenna position; PA1 b=clock bias or electrical path length; and PA1 LOS=line-of-sight, the direction of incident signal arrival. PA1 W=.SIGMA. (i=1 to m) (.theta.i-Hi x+ni) 2/Ri is minimized. W is the "weighted fit error", the sum squared of discrepancies between predicted and actual measurements, weighted by measurement variances. The maximum-likelihood estimate is the (x, n) that minimizes W, and thus is the one sought as the most accurate in evaluating the line-biases.
These articles have presented some approaches for estimating the line-biases for a stationary platform, that offer satisfactory solutions if a suitable test is possible and the line-biases do not change over time. However, as several situations can occur where the test is not possible or the line-biases can be expected to change significantly over time, such as due to changing thermal conditions, these approaches are somewhat limited.
Other articles of interest regarding the GPS in the prior art are:
It will be seen, however, upon studying all of the foregoing literature, that none involves an approach that would produce concurrent generation of attitude parameters, and particularly none using differential phase measurements obtained from an antenna array while performing phase calibration and reliably estimating attitude system line-biases.
Problem to be Solved
It is accordingly a problem in the art to accurately estimate attitude system line-biases and achieve an approach that is effective for concurrent generation of attitude using differential phase measurements obtained from an antenna array while performing phase calibration.
Objects of the Invention
It is therefore an object of the present invention to provide a method and means for producing concurrent generation of attitude using differential phase measurements obtained from an antenna array while performing phase calibration.
It is also an object of the invention to provide a novel approach for accurately estimating attitude system line-biases in performing the phase calibration.